1. Field of the Invention
The present invention relates to a wastewater treatment process for wastewater containing aluminum and manganese, for more details, relates to a wastewater treatment process in which a magnesium oxide having a low content of impurities with high purity is produced from sulfuric acid-acidic wastewater containing magnesium and calcium, and the magnesium oxide is used as a neutralizing agent for a wastewater treatment. The present application claims a priority based on Japanese Patent Application No. 2013-125700 filled on Jun. 14, 2013 in Japan, and the Application is incorporated into the present application by reference.
2. Background Art
In a nickel oxide ore, various components such as cobalt, manganese, magnesium, and aluminum are contained in addition to nickel. In the smelting for the recovery of nickel from a nickel oxide ore, a process called dry smelting in which an oxide ore is put into a furnace together with a reducing agent and roasted has been used in many cases.
In the dry smelting, manganese, aluminum, magnesium, and the like that are not intended to be recovered are effectively separated as slag from nickel and cobalt.
In recent years, a hydrometallurgical process called a HPAL process has also been used, in which a low-grade nickel oxide ore containing only around 1 to 2% by mass or less of nickel, for which smelting cannot economically be performed in the dry smelting described above, is put into a pressurized vessel together with sulfuric acid, and a valuable metal such as nickel is leached into a sulfuric acid solution under an atmosphere of high temperature and high pressure at around 250° C.
For example, as described in Patent Literature 1, the nickel leached in a HPAL process is separated by solid-liquid separation from a slurry composed of leached residues while adding a neutralizing agent, and then into which a neutralizing agent is added to separate the nickel from impurities. Further, the nickel is recovered as a sulfide that is an intermediate material by the addition of a sulfurizing agent into the leached nickel, and separated from the impurity components that are not intended to be recovered, such as aluminum, manganese, and magnesium, which remain in a solution after sulfurization.
By the way, in the solution after sulfurization from which nickel has been separated and recovered, since manganese is present, the solution cannot be discharged as it is to the outside of the system, such as a river, and a sea area. Therefore, the solution after sulfurization is sent to a wastewater treatment step, and into which a neutralizing agent is added to adjust the pH to separate aluminum and manganese as a precipitate, then the waste liquid is intended to be discharged.
However, as compared with the dry smelting process described above, the impurity components are not easily stably fixed in a precipitate, and there may be a case of requiring considerable labor and cost. For example, in order to stably fix the manganese as a hydroxide precipitate, pH of the waste water is required to be adjusted in the range of 8 to 10. Therefore, it is required that as a neutralizing agent, an alkaline slurry of calcium hydroxide or the like is added in a large amount for the neutralization.
Further, magnesium is also contained in the solution after sulfurization described above, however, magnesium does not affect the environment, and is not required to be removed from the wastewater. However, when manganese is treated as described above, magnesium forms a precipitate at the same time of the manganese, therefore, a neutralizing agent in an excess amount to be used for forming the precipitate of the magnesium is required. In such an increase of the amount of the neutralizing agent to be used, the cost is increased, further the amount of the precipitate generated is also increased, and as a result, the labor and cost for the process are increased, and the like, therefore, this is not preferred.
With respect to the problems described above, in Patent Literature 2, a wastewater treatment process in which manganese is selectively removed from the sulfuric acid-acidic wastewater containing aluminum, magnesium, and manganese, which is obtained in Patent Literature 1, has been proposed. This process is a process in which, through a step of adjusting the pH to 4.0 or more to 6.0 or less by the addition of a first neutralizing agent into sulfuric acid-acidic wastewater to separate a solution after dealumination and an aluminum precipitate; a step of adding a slurrying solution into an aluminum precipitate to form a slurry, and then forming an aluminum precipitate slurry after the adjustment of the pH to 9.0 or more to 9.5 or less by the addition of alkali; a step of adding a second neutralizing agent into the solution after dealumination to adjust the pH to 8.0 or more to 9.0 or less, and then adding an oxidizing agent to perform oxidation neutralization and forming a slurry after the oxidation neutralization; and a step of solid-liquid separating the aluminum precipitate slurry and the slurry after the oxidation neutralization; demanganization wastewater is obtained by the suppression of precipitation of magnesium from sulfuric acid-acidic wastewater containing aluminum, magnesium, and manganese, and then a third neutralizing agent is added into the demanganization wastewater to obtain a wastewater precipitate and effluent wastewater.
By using a process described in Patent Literature 2, the amount of a neutralizing agent is reduced, therefore, the cost can be reduced. Further, the generated precipitate is reduced, therefore, the equipment scale can be reduced, and the process is preferred in the environment. In addition, the precipitation of magnesium is suppressed, therefore, the manganese grade in the precipitate is increased, and there are advantages that the cost of reuse by the recycling of manganese can be reduced.
However, in a case of the actual operation in which nickel is industrially recovered from a nickel oxide ore, there were problems that a large amount of neutralizing agent is required, and further when a calcium-based neutralizing agent such as slaked lime, and lime stone that are easily industrially available and commonly used is used, the calcium sulfate (gypsum) itself, which is purified by neutralization, becomes a precipitate, and the effective use of leached residues is limited by the coexistence of the calcium and sulfur that are components of gypsum.
Specifically, in the process of Patent Literature 2 described above, precipitation of calcium is mixed together with the precipitation of manganese, and the manganese becomes difficult to be reused as a resource, and not only cannot effectively be utilized as a valuable resource, but the manganese precipitate is accumulated to store or discarded, accordingly, the measure for the environmental protection is required.
In this point, salts having high solubility is considered to be used as a neutralizing agent, and in this application, magnesium salts such as the magnesium oxide described above, and magnesium hydroxide are suitable. Therefore, in a hydrometallurgical process for a nickel oxide ore, an attempt to recover magnesium from the wastewater generated in a process and to utilize the magnesium oxide as a neutralizing agent has also been made.
Specifically, for example, as one of the process for recovering magnesium from the solution to be sent to a wastewater treatment, there is a process described in Patent Literature 3. In Patent Literature 3, a process for recovering a magnesium oxide from a source of a magnesium sulfate, including: a step of preparing a source of a magnesium sulfate in a solution state, which is obtained from the part of the process relating to the leaching of an ore containing metal or a concentrate; a step of converting the magnesium sulfate in a solution state into a magnesium sulfate in a solid state; a step of bringing the magnesium sulfate in a solid state into contact with elemental sulfur in a reducing atmosphere; and a step of recovering the magnesium as a magnesium oxide, and the sulfur as sulfur dioxide gas; has been proposed.
However, in this process of Patent Literature 3, in order to recover the crystal of magnesium sulfate from the wastewater after the separation of valuables such as nickel, a process in which the decrease of the solubility under high concentration of sulfuric acid is utilized to perform crystallization and dehydration by the contact with the concentrated sulfuric acid produced from the sulfur dioxide gas is used. In this case, magnesium remaining without being crystallized is repeatedly used in the leaching step together with sulfuric acid, the amount of the crystallized magnesium depends on the amount of the sulfuric acid to be used for the leaching, therefore, it is not easy to maintain the balance. In particular, in a case where the magnesium is separated to be used for a neutralizing agent, or in a case where the amount of the magnesium contained in an ore is large, there is a problem that the flexibility of the operation is limited.
Further, in Patent Literature 4, a leaching process in the air in the recovery of nickel and cobalt from a laterite ore, including: a step of separating a laterite ore into an ore fraction containing low magnesium and an ore fraction containing high magnesium by selection mining or subsequent separation; a step of slurrying the separated ore fractions separately; a step of leaching an ore fraction containing low magnesium by using concentrated sulfuric acid as the primary leaching step; and a step of introducing an ore slurry containing high magnesium after the primary leaching step and the completion of the iron precipitate as another low-sulfur containing form of a goethite, an iron oxide, or an iron hydroxide, and leaching an ore fraction containing high magnesium by using the sulfuric acid liberated in an iron precipitate as the secondary leaching step; has been proposed.
By using such a process, it is also considered that the magnesium contained in a nickel oxide ore is used as a neutralizing agent, or magnesium is recovered from a solution after neutralization and can repeatedly be used as a neutralizing agent.
However, in a case of using these process, an enormous amount of heat energy is required when magnesium is concentrated from a large amount of wastewater, and further there is a concern that impurities contained in a mineral are accumulated in a process with the repeated use of the neutralizing agent.
In addition, it is common that the grade of the magnesium contained varies depending on the kind of the mineral, the mining place, or the time of mining, and is not stable. Therefore, if the magnesium is insufficient, it is considered to use a calcium-based neutralizing agent that is inexpensive and can stably be supplied, such as conventional slaked lime and lime stone, in combination. However, in this case, as in the conventional process described above, calcium is also introduced into a process, and circulated in the process. Further, when magnesium is tried to be recovered from wastewater, part of calcium behaves as in magnesium, therefore, magnesium cannot be used for an application other than that of the neutralizing agent.
As a process for separating magnesium and calcium in a solution, for example, there is a process shown in Patent Literature 5. In the process described in Patent Literature 5, magnesium hydroxide is recovered from the waste liquid containing a large amount of magnesium sulfate, which is discarded and discharged in a flue gas desulfurization plant in which a magnesium hydroxide is used as a desulfurizing agent, and circulated into a flue gas desulfurization step, therefore, the process contributes to the recycling and the environmental cleanup. Specifically, ammonia is added into the flue gas desulfurization wastewater containing a magnesium sulfate to form a precipitate of a magnesium hydroxide, and into the resultant solution, milk of lime is added to form a calcium sulfate and ammonia, and the ammonia is circulated in the step. The magnesium hydroxide obtained as described above is slurried by the final waste liquid of the present process, and by the circulation of the slurry into a desulfurization plant, complete circulation of wastewater in the desulfurization plant is realized, as a result, the elimination of the discarding and discharging of wastewater can be realized. In addition, a washing step is provided to improve the purity of the obtained calcium sulfate, and as a result, the advantage of outside sales can be enhanced.
However, in the process described in Patent Literature 5, since ammonia is used, a complicated equipment is required, further there is a problem that the investment and operation cost are increased, and the process is difficult to be easily used. Further, as described above, when the magnesium hydroxide and magnesium oxide produced from the magnesium components contained in a nickel oxide ore are tried to be used as a neutralizing agent, it is unavoidable to be expensive in the cost as compared with lime stone or slaked lime, and it is not practical to cover all the water-soluble neutralizing agent with the magnesium hydroxide and magnesium oxide produced above. In addition, there may also be an influence of the calcium component and the like contained in an ore or in the impurities that are treated at the same time.
Therefore, in Patent Literature 6, a process for adjusting magnesium oxide from a metal sulfate solution containing magnesium sulfate and calcium is shown. This process is a process in which metals other than the magnesium are precipitated as a hydroxide to perform solid-liquid separation, the separated solution is concentrated so that the specific gravity is in the range of 1.35 to 1.5 and a calcium sulfate is separated, a magnesium sulfate is recovered from the solution after the separation, and a magnesium oxide is recovered by thermal decomposition.
However, in the process described in Patent Literature 6, there is a problem that when the concentration is advanced in order to separate the calcium sulfate, part of the magnesium is also precipitated together with calcium, and the recovery efficiency is lowered. This is because when a compound of a calcium sulfate dihydrate is precipitated, a magnesium sulfate heptahydrate is started to be precipitated in parallel, and in order to separate both, various process such as a process for analyzing a component of a solution, a process for observing the apparent difference with the naked eye, or a process for measuring the specific gravity can be used, however, it takes a lot of labor.
As described above, in the conventional process, it is not easy to obtain a magnesium oxide having a low content of impurities with high purity efficiently and at a low cost, and in a case where the magnesium oxide is used for a wastewater treatment as a neutralizing agent, the grade of the manganese separated from wastewater is impaired, and further as a result, the cost required for the separation of the manganese is also increased.